Minimum Temperatures in Big Bear Basin, California RICHARD A. MINNICH9 This investigation developed from an interest in winter temperatures in Big Bear Basin; they seemed unduly extreme in the Southern California environment. An examination of many years of simple climatological data revealed that recent minimum temperatures appeared to be warmer than in previous years and that variability of rainfall caused fluctuations in the areal extent of Big Bear Lake. An extended period of subnormal precipitation between 1946 and 1965 resulted in unusually low levels of Big Bear Lake, the lowest being less than five percent capacity in the winter of 1961-1962 (see Figure 1 ). Since the fall of 1965 a series of flood-producing storms has rapidly enlarged the lake so that currently it is roughly fifteen times as large by surface area (Figure 2) as it was in 1961-1962. This set of circumstances makes possible the present study on the effect of the lake as an ameliorating water body upon the nighttime temperatures. It was the purpose of this investigation to demonstrate that with lake enlargement, the extreme nocturnal cold has been modified importantly , perhaps as much as five Fahrenheit degrees. The Big Bear Basin (Figure 3) is part of the Santa Ana River drainage in the San Bernardino Mountains, the easternmost extension of the Transverse Ranges of Southern California. It is approximately one hundred miles east of Los Angeles. The lake at maximum has a surface area of six square miles, which is more than half the area of the sediment floor. The basin is about 6,750 feet high and is bounded by east-west trending ridges 8,000 feet in elevation which materially reduce wind velocities over the basin. * Mr. Minnich was a graduate student in geography at the University of California, Riverside 92502, when this paper was recognized as the Outstanding Student Paper presented at the 1968 meeting of the Association in Bellingham. 83 84ASSOCIATION OF PACIFIC COAST GEOGRAPHERS Figure 1. Looking east along the axis of Big Bear Basin from Big Bear Dam in July 1960 when the lake level was low. Ridges approximately 8,000 feet high north and south of the basin reduce wind flow. A layer of cold air forms over the sediment floor on clear calm nights due to surface-induced radiational cooling. Figure 2. Same area as Figure 1 early in the morning of January 24, 1968, when the lake was near capacity. Note layers of haze over frozen lake surface in the distance. While temperatures on this date were as high as 46 degrees F on nearby slopes, they were consistently about 20 degrees F on the basin floor. YEARBOOK · VOLUME 33 · 197185 As usual for mountain areas, climatic data are meager and vertical cross sections of the local air column never have been attempted in the Big Bear area, either from soundings or by transects of automatic recording stations along adjacent mountain slopes. Therefore this study has been based on simple climatological data. To obtain a measure which could demonstrate lake influence on temperature during nighttime hours, a rather simple statistical method was devised —examination of the daily range at Big Lake Fire Department for which data are published in Climatological Data, California.1 The reduction of daily range of temperature observed with increase of lake size is believed to have taken place within a locally controlled ground inversion. There are many implicit evidences of such a control : (1) the minimum temperatures are unusually low for a Southern California mountain station, (2) the structure of the basin is well suited for its formation, (3) explanation for changes in temperature as radical as those observed within a short time span is more feasible within a "cell" of air of limited areal extent made relatively autonomous from the remainder of the atmospheric column by an inversion , and (4) there are certain peculiar vegetation patterns, namely the basin floor is dominated by Great Basin sage species. California black oak ( Quercus kelloggii), the only deciduous oak in Southern California, is highly sensitive to freezing temperatures, is conspicuously absent from the bottom 200 feet of the valley by elevation , but is found on peaks and adjacent slopes up to elevations...
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Apr 1, 1975
A Mani + 1
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